1. Field of the invention
The present invention relates to a data transmission system for transmitting data among host CPU (central processing unit) and various slave, or object, CPUs.
2. Description of the Prior Art
Nowadays, machines have become sophisticated and are controlled by not only one microcomputer but by a plurality of microcomputers, called slave microcomputers, for controlling various different operations. For controlling the overall operation of the machine in a predetermined timed relationship, a host microcomputer is connected to the slave microcomputers. The host microcomputer and slave microcomputers define a total control system for controlling the machine operation.
Each of the slave microcomputers and host microcomputers can be defined by a CPU and other elements, such as a memory and an input/output (I/O) section. These elements, including the CPU, defining a microcomputer can be included on a single semiconductor chip and, therefore, no external data line is needed to transfer data within one chip of microcomputer. But, when a host microcomputer and slave microcomputers are employed in a single machine, data must be transferred among these microcomputers, and for this purpose, external data lines must be extended between the host microcomputer and each slave microcomputer, resulting in many data lines, the number of which is equal to the number of slave microcomputers employed. Furthermore, when it is required to add a slave microcomputer, such as when an optional device is to be added to the machine to upgrade the machine, it is very difficult to change the data transmission system. In other words, once the data lines are extended and programming for the data transmission is well organized, it is very difficult to add one or more slave microcomputers to the established control system. The reason being that it is necessary to increase the number of input/output ports in the host microcomputer for the additional slave microcomputer and to add another data line extending between the host and the newly added slave microcomputer. Furthermore, the programming for the old data transmission must be changed to accommodate the new slave microcomputer.
An approach has been made to improve the above described data transmission system between a host microcomputer and slave microcomputers, and is disclosed, e.g., in Tokkaisho (Japanese Patent Laid-open Publication) No. 57-64749 published Apr. 20, 1982. According to Takkaisho No. 57-64749, one data bus structure extends from the host microcomputer, and slave microcomputers are connected to the data bus structure through respective branch lines. Futhermore, a plurality of pairs of lines are needed, each pair extending between the host microcomputer and one slave microcomputer. Therefore, the data transmission system of Tokkaisho No. 57-64749 has one data bus structure with a number of branches equal to the number of slave microcomputers, and a number of pairs of lines also equal to the number of slave microcomputers. The data transmission system of Tokkaisho No. 57-64749 operates as follows.
The host microcomputer produces a request signal REQO which is sent through one line of a particular pair of lines extending between the host microcomputer and a designated object slave microcomputer, thereby transmitting the request signal REQO to the designated object slave microcomputer. Upon receipt of the request signal REQO, the object slave microcomputer enables its I/O section and, at the same time, sets its data DATA-A to its I/O register. When this is done, the object slave microcomputer produces a response signal REQE which is sent back to the host microcomputer through the other line of the particular pair of lines between the host and object slave microcomputers. Then, upon receipt of the response signal REQE, the host microcomputer sends out its data DATA-B through the bus structure to every one of the slave microcomputers, but only the object slave microcomputer accepts it. Furthermore, during, or after, the receipt of the data DATA-B from the host microcomputer, the object slave microcomputer sends out its data DATA-A through the bus structure to the host microcomputer, thereby completing a data exchange between the host and designated slave microcomputer.
Although the data transmission system according to Tokkaisho No. 57-64749 has reduced the number of data lines by the employment of a single data bus structure, it still requires a number of pairs of lines extending between the host and each of the slave microcomputers for the exchange of request and response signals. Therefore, when it is required to add another slave microcomputer, it is not only necessary to add another pair of lines, but it is also necessary to increase the number of I/O ports in the host microcomputer for the connection of another pair of lines. Furthermore, programming in the host microcomputer must be changed for enabling the designation of the newly added slave microcomputer.